1. Field of the Invention
This invention generally relates to recycled rubber products and more particularly to composites made from ground rubber particles and polyurethane binders.
2. Description of the Related Art
Old tires and other rubber articles may be ground into rubber particles suitable for recycling. This ground rubber is also known as crumb rubber. At the present time in the United States, Canada, and Mexico, over 20 million pounds of recycled rubber are used each month. Recycled rubber is used to produce many finished goods, including welcome mats, animal mats, truck-bed liner mats, playground mats, speed bumps, railroad crossing pads, running tracks, and bases for traffic control devices. Presently, finished goods produced from recycled rubber are made by either vulcanizing the rubber or by making composites using ground rubber and polyurethane binders.
In the vulcanizing process, the rubber is first dried to reduce the water content to less than 0.5%. The rubber is then vulcanized with sulfur. In some cases, the rubber is first treated to give a product with higher tensile strength. Rubber treated for vulcanization cannot be used in the polyurethane binder process.
Many types of binders are used in the polyurethane binder process. The terms xe2x80x9cpolyurethane binderxe2x80x9d and xe2x80x9cbinderxe2x80x9d are used interchangeably herein. The cost of these binders ranges from about $0.70 per pound to about $1.40 per pound. In the commercial process, the ground rubber is mixed with from about 3% to 20% of the binder, and the mixture is molded into the finished product. The process may be run under a variety of conditions, for example high heat and high pressure or low heat and low pressure. Dry rubber, rubber containing less than 0.5% water, is desirable, but not necessary. A finished product made with rubber with a high water content has lower tensile strength than one made with dry rubber, however. Most commercial crumb rubber contains more than 1% water, and few manufacturers dry the rubber.
The preferred embodiments of the invention provide methods of making composites from ground rubber and polyurethane binders. In one aspect, such methods use less binder than the conventional polyurethane binder method, while providing products having high tensile strengths. The reduced amount of binder also reduces the curing time.
A second embodiment relates to a method of treating rubber particles with polyurethane. These treated rubber particles are used to make molded composite products.
A third embodiment relates to recycled rubber products made from ground rubber and polyurethane binders. These products have high tensile strength compared to similar products made with the conventional process using the same proportion of rubber and binder.
A fourth embodiment relates to polyurethane-coated rubber particles. Additional binder may be added to these coated or treated rubber particles to make recycled rubber products. A pigment or dye may be added in the coating step to make colored rubber particles. These particles are also suitable for loose-fill applications.